The study of the relationship of three dimensional structure of organic or inorganic molecules is called stereochemistry, one aspect of which is the study of stereoisomers. Isomers are compounds which have the same molecular formula; stereoisomers differ from each other only in the way the atoms are oriented in space. Enantiomers are stereoisomers which are mirror images of each other and which share all of the same physical properties except one: the direction in which they rotate a plane polarized light, or optical rotation. Enantiomers are distinguishable from each other only by their optical rotation. Enantiomers display a property known as chirality. One characteristic of chiral molecules is that they are not superimposible on their mirror images. One enantiomer rotates the plane of polarized light to the left and the other enantiomer, its mirror image, rotates the plane of light to the right. If the rotation of the plane is to the right (clockwise), the substance is dextrorotatory, and the enantiomer is designated with a plus (+) sign. Conversely, if the optical rotation is to the left (counterclockwise), the substance is levorotatary, and the enantiomer is designated as minus (-). Enantiomers display equal, but opposite, optical rotation.
The existence of stereoisomers provides us with one of the most sensitive probes into mechanisms of chemical and biological reactions. Despite the very close similarity of enantiomers, they may display very different properties in chemical and biological reactions or transformations. Although very similar in structure, one isomer of the chiral pair may serve as an antibiotic, heart stimulant, food, or display other biological activity, and the other isomer may be totally biologically inert or has the opposite and toxic effect. J. March, Advanced Organic Chemistry, page 87, (2nd edition) McGraw Hill Co., New York, N.Y. (1977).
Enantiomers have identical chemical properties, except their interactions with optically active reagents. Enantiomeric molecules undergo the same chemical reactions with achiral molecules, and at the same rates, since the atoms of each are influenced in their reactivity by exactly the same combination of substituents. A reagent approaching either enantiomer encounters the same environment, except that one environment is the mirror image of the other. In the case of an optically active reagent, however, the influences exerted on the reagent in its attack on the enantiomers are not identical, and reaction rates will be different. In some systems of this nature, the reaction will not take place at all for one of the isomers.
In biological systems, such stereospecificity is the rule rather than the exception. The most obvious reason for this is that enzymes and most of their targets are optically active molecules. For example, (+)-glucose is an extremely important metabolite, but (-)-glucose is biologically inactive. The hormonal activity of (-)-adrenaline is several times greater than its mirror image (+)-adrenaline, and only one enantiomer of the compound chloromycetin is an antibiotic. (For other examples see R. T. Morrison and R. N. Boyd, Organic Chemistry, 3rd edition, Allyn and Bacon, Inc., Boston, Mass., pages 126-127) This stereospecific activity is one reason why chiral molecules are so important as a basis for bioactive compounds.
Tartaric acid, (C.sub.4 H.sub.6 O.sub.6) which is readily available in both enantiomeric forms, is an extremely useful and versatile chiral starting material for natural product and drug synthesis. Tartaric acid is used extensively in industry and as a food acidulant. Its two-fold symmetry axis causes all four functionalized carbon atoms to exist as homotopic pairs and thus simplifies its functional group transformations. Owing to its unique properties, many synthetic applications based on tartaric acid have been developed. For example, D. Seebach and coworkers describe the synthesis of chiral compounds from tartartic acid enantiomers in Modern Synthetic Methods 1980, R. Scheffold (ed.), Otto Salle Verlag, Frankfurt, FRG, p. 115-171 (1980). Hungerbuhler et al. describe the synthesis of chiral agents from tartaric acid using an epoxyalcohol intermediate in Angew. Chem. Int. Ed. Engl., 18(12):958-960 (1979). Schnurrenberger, et al. describe the synthesis of a chiral antibiotic, (+)-colletodiol, in Liebigs. Ann. Chem., pages 733-744 (1987). Mori and Iwasawa describe the synthesis of chiral pheromones using the enantiomers of tartrate as starting materials in Tetrahedron, 36:87-90(1980). Saito et al. describe the production of four stereoisomers of pure 2,3-epoxyesters from tartaric acids in Tetrahedron Letters, 27(43):5249-5252 (1986). Hansson and Kihlberg describe the synthesis of an optically active derivative of the amino acid aspartic acid in J. Org. Chem., 51:4490-4492 (1986).
Other cyclic systems are described by Guiller, A. et al. in Tetrahedron Lett. 1985, p. 6343; Tewson, T. J. and M. J. Welch in J. Nucl. Med., 21:559 (1980); David, S. and S. Hanessian, Tetrahedron, 41:643 (1985); Ricci, A. et al. in J. Chem. Soc. Chem. Commun., p. 1458 (1985); and Shanzer, A., Tetrahedron Lett. 1980, p. 221 (1980).